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8 How Cells Divide PowerPoint® Lectures prepared by Johnny El-Rady Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8.1 Prokaryotes Have a Simple Cell Cycle Cell division in prokaryotes takes place in two stages The DNA is replicated The cell elongates, then splits into two daughter cells The process is called binary fission Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Cell division in prokaryotes Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8.2 Eukaryotes Have a Complex Cell Cycle Cell division in eukaryotes is more complex than in prokaryotes because 1. Eukaryotic contain far more DNA 2. Eukaryotic DNA is packaged differently It is in linear chromosomes compacted with proteins Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8.2 Eukaryotes Have a Complex Cell Cycle Eukaryotic cells divide in one of two ways Mitosis Occurs in somatic (non-reproductive) cells Meiosis Occurs in germ (reproductive) cells Results in the production of gametes Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The complex cell cycle of eukaryotic cell is composed of several stages G1 phase Primary growth phase Interphase S phase DNA replication G2 phase Microtubule synthesis M phase Mitosis Chromosomes pull apart Cytokinesis C phase Cytoplasm divides Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mitosis Cell cycle – events between one cell division and the next 2 major stages Interphase – cell not dividing but cell very active Cell division – mitosis and cytokinesis Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8.3 Chromosomes The number of chromosomes varies enormously from species to species The Australian ant Myrmecia spp. has only 1 pair Some ferns have more than 500 pairs Chromosomes exist in somatic cells as pairs Homologous chromosomes or homologues Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Diploid cells have two copies of each chromosomes Replicated chromosomes consist of two sister chromatids These are held together at the centromere Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8.3 Chromosomes Humans have 46 chromosomes The 23 pairs of homologous chromosomes can be organized by size This display is termed a karyotype Fig. 8.4 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8.3 Chromosomes Chromosomes are composed of chromatin Complex of DNA (~ 40%) and proteins (~ 60%) A typical human chromosome contains about 140 million nucleotides in its DNA This is equivalent to About 5 cm in stretched length 2,000 printed books of 1,000 pages each! In the cell, however, the DNA is coiled Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8.3 Chromosomes The DNA helix is wrapped around positivelycharged proteins, called histones Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8.4 Cell Division The eukaryotic cell cycle consists of the following stages Interphase Mitosis Division of the nucleus Subdivided into Prophase, metaphase, anaphase, telophase Cytokinesis Division of the cytoplasm Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Review the Concepts Prokaryotic cells divide by which cell cycle? What are collectively known as G1, S, and G2 phases of the cell cycle? During which phase of the cell cycle is DNA synthesized? What are called the proteins with positive charges that are wrapped around DNA? Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Interphase Chromosomes replicate and begin to condense Mitosis Prophase Nuclear envelope breaks down Chromosomes condense further Spindle apparatus is formed Metaphase Chromosomes align along the equatorial plane Spindle fibers attach at the kinetochores On opposite sides of the centromeres Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 8.8 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mitosis Anaphase Sister chromatids separate They are drawn to opposite poles by shortening of the microtubules attached to them Telophase Nuclear envelope reappears Chromosomes decondense Spindle apparatus is disassembled Cytokinesis Two diploid daughter cells form Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 8.8 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Cytokinesis Animal cells Cleavage furrow forms, pinching the cell in two Fig. 8.8 Plant cells Cell plate forms, dividing the cell in two Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Cell Death During fetal development, many cells are programmed to die Human cells appear to be programmed to undergo only so many cell divisions About 50 in cell cultures Only cancer cells can divide endlessly Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8.5 Controlling the Cell Cycle The eukaryotic cell cycle is controlled by feedback at three checkpoints Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8.5 Controlling the Cell Cycle 1. Cell growth is assessed at the G1 checkpoint G0 is an extended rest period 2. DNA replication is assessed at the G2 checkpoint 3. Mitosis is assessed at the M checkpoint Fig. 8.11 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8.6 What is Cancer? Cancer is unrestrained cell growth and division The result is a cluster of cells termed a tumor Benign tumors Encapsulated and noninvasive Malignant tumors Not encapsulated and invasive Can undergo metastasis Fig. 8.13 Leave the tumor and spread throughout the body Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 8.8 Curing Cancer Potential cancer therapies are being developed to target seven different stages in the cancer process Stages 1-6 Prevent the start of cancer within cells Focus on the decision-making process to divide Stage 8 Act outside cancer cells Prevents tumors from growing and spreading Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 8.15 New molecular therapies for cancer Stopping tumor growth Receiving the signal to divide Stepping on the gas Passing the signal via a relay switch Amplifying the signal Releasing the “brake” Checking that everything is ready Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display